The high strength bolt is the load that can be endured is larger than the ordinary bolt of the same specification. Common bolts are made of Q235 (A3). The material of high strength bolt is 35 # steel or other high quality material, which is heat treated after being made to improve the strength. The difference between them is the strength of the material.
From the view of raw materials: high strength bolts are made of high strength materials. The bolts, nuts and washers of high strength bolts are made of high strength steel, commonly used in 45 steel, 40 boron steel, 20 manganese titanium boron steel, 35CrMoA, etc. Common bolts are usually made of Q235 (equivalent to the previous A3) steel. From the strength level, high-strength bolts are used more and more widely. 8.8s and 10.9s strength grades are commonly used, of which 10.9 grade is the most. The strength grade of ordinary bolts is lower, generally 4.4, 4.8, 5.6 and 8.8.
From the force characteristics, high strength bolts exert pre-tension and transfer external force by friction. Ordinary bolt connection relies on bolt shear and hole wall pressure to transmit shear force. Pre-tension force produced when screwing nuts is very small and its influence can be neglected. In addition to high material strength, high-strength bolts also exert a large pre-tension force on bolts, which causes extrusion force between connecting components, so that the direction perpendicular to the bolt is very large. Friction, and pre-tension, anti-slip coefficient and steel types have a direct impact on the bearing capacity of high-strength bolts. Minimum specification M12 of high strength bolts, commonly used M16 ~ M30, the performance of super-large bolts is unstable, and should be carefully used in design. The difference between high strength bolt friction connection and compression connection: high strength bolt connection can be divided into high strength bolt according to different design and stress requirements when shearing force is applied. There are two kinds of bolt friction connection and high strength bolt pressure connection. The essential difference between them is the difference of the limit state. Although they are the same bolt, they are quite different in calculation method, requirements and application scope.
From the use point of view: ordinary bolts can be reused, high-strength bolts can not be reused. High strength bolts are generally used for permanent connections. The high-strength bolt is a pre-stressed bolt. The friction type uses a torque wrench to exert the prescribed prestressing force, and the pressure type twists off the plum blossom head. Common bolts have poor shear resistance and can be used in minor structural parts. Ordinary bolts only need to be tightened. The mechanical performance and calculation method of common bolt and high strength bolt are different. The force of high-strength bolt is to bear external load by applying pre-tension P in its interior first, and then generating friction resistance on the contact surface between the joints, while ordinary bolt bears external load directly.
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